1. Field of the Invention
The present invention relates to a method of measuring frequency selectivity of the sense of hearing, and a method of, and an apparatus for, estimating the shape of a human auditory filter by the frequency selectivity measurement method.
2. Description of the Relevant Art
Hearing characteristic tests for hearing impairment that are most frequently conducted nowadays are a hearing test (measurement of audiogram) and a speech intelligibility test. In the hearing test, it is possible to ascertain frequency characteristics of the threshold of hearing of the hearing impairment. In the speech intelligibility test, it is possible to determine any ability of discrimination of speech by the hearing impairment.
However, the hearing characteristics of the hearing impairment vary with individuals. It is therefore considered that these two methods can only grasp a part of the complicated hearing characteristics.
It is generally said that the shape of audiogram and ability of discrimination of speech has not only deteriorated, but their frequency selectivity has deteriorated in impaired ears. The frequency selectivity is the capability of perceiving two sounds that are different in frequency. Normal hearing can perceive two different sounds that are close in frequency, for example, 1 kHz v. 1.2 kHz, but the hearing impairment whose frequency selectivity has deteriorated cannot perceive these two different sounds.
When the extent of deterioration in the frequency selectivity becomes greater, it leads to deterioration of ability of discrimination of speech or deterioration of ability of discrimination of speech sound under noisy conditions. It is therefore very useful to know the extent of deterioration in the frequency selectivity for the hearing impairment""s diagnosis, a grasp of the hearing impairment""s hearing characteristics, hearing aid fittings, etc.
In recent years, an auditory filter has been suggested as a model for expressing a mechanism of frequency analysis of the human sense of hearing. This is considered to be a method for expressing a mechanism of frequency analysis of the human inner ear by band-pass filter banks. The shape of each filter (auditory filter) within these filter banks is usually measured by notched noise masking. A simplified method for measuring the frequency selectivity for the hearing impairment that uses a theory of this auditory filter is disclosed in Japanese patent No. 2723780. It is known that a roex (p, r) filter can be used as a model for the shape of a human auditory filter.
In the measurement of the auditory filter by notched noise masking, it is said that the shape of each subject""s auditory filter can be measured with high accuracy. However, this measurement takes a long time and it is practically impossible to measure the shape of the auditory filter of a hearing impairment individual at any time by clinicians or audiologists. Japanese patent No. 2723780 suggests a method of measuring whether or not frequency selectivity has deteriorated in a short time. It is possible to judge whether there is any deterioration in the frequency selectivity, but is not possible to measure the extent of deterioration in the frequency selectivity, or the shape itself of the auditory filter.
The hearing threshold level of the hearing impairment naturally rises higher than that of normal hearing and if noise is superposed thereon, the threshold of detection level of a tone (probe signal) rises further. In addition, if that hearing impairment""s frequency selectivity has deteriorated, the noise""s influence on the individual is much greater than for normal hearing. Therefore, in many cases, the hearing impairment whose hearing level has severely deteriorated cannot detect a tone even though the tone level has reached the maximum output level of an inspection instrument (off-the-scale).
It is also said that the shape of the auditory filter varies with the level of the input signal. In the measurement of the auditory filter of normal hearing, it is said that a masker (notched noise) whose level is about 40 dB/Hz is most suitable judging from the information in the past. It is also said that when measured at a level higher than 40 dB/Hz, the shape of the auditory filter becomes broad. However, such a shape change in the auditory filter as seen in the normal hearing does not always occur in all hearing impairment cases according to their levels. There is no method of measuring the shape change characteristics of the auditory filter according to each level of the hearing impairment to date.
The primary object of the present invention is to overcome the above-mentioned drawbacks and to provide a method of measuring frequency selectivity comprising the steps of adding notched noise with a predetermined notch width to a detection sound which sets frequency and sound pressure level at a predetermined value to produce an inspection sound, transmitting the inspection sound to a subject, judging whether or not the subject can perceive the detection sound from the inspection sound by widening the notch width, and obtaining the threshold notch width for the subject.
With this method, the frequency selectivity of the hearing impairment subject whose power of hearing has highly or seriously deteriorated and whose threshold of hearing is high can also be measured by the inspection instrument without going off the scale.
Further, if the notch width is set at the threshold notch width of normal hearing, it is possible to easily judge whether the frequency selectivity of a subject has deteriorated further than that of normal hearing.
Another object of this invention is to provide a method of measuring frequency selectivity which comprises the steps of adding notched noise with a predetermined notch width to a detection sound which sets frequency and sound pressure level at a predetermined value to produce an inspection sound, transmitting the inspection sound to a subject, judging whether or not the subject can perceive the detection sound from the inspection sound by narrowing the notch width, and obtaining the threshold notch width of the subject.
With this method, the frequency selectivity of the hearing impairment subject whose power of hearing has highly or seriously deteriorated and whose threshold of hearing is high can also be measured without going off the scale of the inspection instrument.
A further object of this invention is to provide a method of measuring frequency selectivity which comprises the steps of adding noise with a predetermined notch width generated from white noise which is set at a threshold masking level, to a detection sound which sets frequency and a sound pressure level to predetermined values to produce an inspection sound, transmitting the inspection sound to a subject, judging whether or not the subject can perceive the detection sound from the inspection sound by widening a notch width, and obtaining the threshold notch width of the subject.
A further object of this invention is to provide a method of measuring frequency selectivity comprising the steps of adding noise with a predetermined notch width generated from white noise which is set to a threshold masking level to a detection sound which sets frequency and a sound pressure level at predetermined values to produce an inspection sound, transmitting the inspection sound to a subject, judging whether or not the subject can perceive the detection sound from the inspection sound by narrowing a notch width, and obtaining the threshold notch width of the subject.
With this method, it is possible to easily measure the frequency selectivity of the power of hearing.
If the sound pressure level of the detection sound is set to the threshold of hearing of the subject or to a sound pressure level which adds a predetermined level to the threshold of hearing, it is possible to easily judge whether or not the subject could detect the detection sound signal. With the hearing impairment subject whose power of hearing has highly or seriously deteriorated and whose threshold of hearing is high, it is possible to measure the frequency selectivity of the subject""s sense of hearing using the inspection instrument.
Another object of this invention is to provide a method for estimating the shape of an auditory filter by finding a coefficient p of a roex (p, r) filter used as a model for the shape of the auditory filter, comprising the steps of determining a threshold masking level Nx based on a tone S which adds a given value x to a threshold of hearing T of a subject in frequency f, generating a tone Sxe2x80x2 which deducts a given value a from the tone S, generating a masker M with a notch width g and a level Nx including the frequency f in the notch, transmitting to a subject an inspection sound which superposes the masker M on the tone Sxe2x80x2, measuring a minimum notch width gxxe2x88x92a of the subject by changing the notch width g, calculating a coefficient p from the minimum notch width gxxe2x88x92a and the given value a, and estimating the shape of the auditory filter by the coefficient p and the given value x corresponding to a coefficient r.
With this method, it is possible to estimate the effective shape of the auditory filter to efficiently and accurately conduct a diagnosis of the hearing impairment, to obtain a grasp of the hearing characteristics of the hearing impairment, of hearing aid fittings, etc.
By using the given value x as a parameter, when the shape of the auditory filter is estimated from the coefficient p calculated from the minimum notch width gxxe2x88x92a and the value a and the value x corresponding to the coefficient r, it is possible to estimate the shape of the auditory filter corresponding to the input signal. It is also possible to know the relationship between the input signal level and the auditory filter.
A further object of this invention is to provide an apparatus for estimating the shape of an auditory filter by finding a coefficient p of a roex (p, r) filter used as a model for the shape of the auditory filter, comprising a tone generation element for generating a tone with a predetermined frequency, a tone level setting element for amplifying or attenuating the tone generated at the tone generation element to a predetermined level;
a noise generation element for generating noise without a notch, a noise level setting element for amplifying or attenuating the noise generated at the noise generation element to a predetermined level, a notch width setting element for adding a notch including frequency of the tone to the noise, a notched noise superposition element for superposing notched noise output from the notch width setting element on a tone output from the tone level setting element, an inspection sound transmission element for transmitting an inspection sound output from the notched noise superposition element to a subject, and an auditory filter calculation and indication element for calculating a coefficient p of the roex (p, r) filter based on a notch width at a point where the subject can perceive the inspection sound and for indicating the filter shape from the coefficients p and r obtained.
With this apparatus, it is possible to estimate the shape of the effective auditory filter to efficiently and accurately conduct a diagnosis of the hearing impairment, to obtain a grasp of the hearing characteristics of the hearing impairment, of hearing aid fittings, etc.
Further, when the threshold masking level Nx and/or the minimum notch width gxxe2x88x92a, are measured, if transmission of the tone is started at predetermined time intervals after starting transmission of the masker to the subject, it is possible to easily judge the validity of the subject""s response from the timing of tone transmission and the timing of the response by the subject.